In general, structures such as an illumination optical system that radiates illumination light onto an imaging subject, an observation optical system for observing the imaging subject, a channel for guiding a treatment tool or the like, a nozzle or the like for washing the lens surface of the observation optical system, and so on, are disposed at a distal-end portion of an endoscope apparatus. In addition, an endoscope apparatus that efficiently irradiates a viewing field having a wide viewing angle by having a plurality of illumination optical systems disposed therein has been proposed. In this case, because the individual structures need to be efficiently disposed in a limited space, the illumination optical systems are disposed so as to flank the observation optical system.
Also, an endoscope apparatus with which it is possible to perform normal observation and near-field observation by changing the focal distance by moving some of the lenses in the observation optical system has been proposed.
As an example of such an endoscope apparatus, Patent Literature 1 discloses an endoscope apparatus with which, by setting the operating range for achieving the best focusing position within an area in which illumination beams emitted from a plurality of illumination windows overlap, it is possible to illuminate an imaging subject with sufficient brightness and with small illumination unevenness during magnifying near-field observation.
In addition, Patent Literature 2 discloses an endoscope apparatus with which the variability in the observation regions during near-field observation is minimized by providing an illuminating means at a position at which illuminances of peripheral portions become equal to or less than twice the illuminances at center portions of observation regions on either side of the observation optical system.
Furthermore, Patent Literatures 3 to 5 disclose endoscope apparatuses that are provided with, in the distal-end surface of an inserted portion, an observation window of an observation optical system, a forceps port having a circular shape in which the diameter thereof is greater than that of the observation window, and a plurality of light radiation windows through which illumination light is radiated, wherein the plurality of light radiation windows are disposed so as to flank the observation window.